1. Field of the Invention
The present invention relates to an automatic equalizer provided on a data transmission channel for regenerating an original waveform from digital data and, more particularly, to an automatic equalizer applicable to the reception or transiting of digital data which is transmitted at high speed over a telephone line, radio channel or similar data transmission channel.
2. Description of the Prior Art
When digital data is transmitted over a telephone line, for example, it is likely that the waveform representative of the data is sequentially distorted due to the fluctuation of a power source located at a transmitting station and/or the frequency characteristic particular to the telephone line and cannot be accurately demodulated at a receiving station. Generally, this kind of distortion is ascribable to intersymbol interference, phase jitter, and frequency offset. The intersymbol interference results from reflected waves which occur at, for example, the branching points of the transmission channel and interfere with the original waveform. The phase jitter is caused by the fluctuation of the phase of the data wave while the frequency offset is a frequency deviation appearing in the substantially constant movement of the phase of the data wave. The phase jitter and frequency offset result in the fluctuation of the phase of the received waveform. In light of this, it is a common practice to provide a receiving station and a tandem station with a demodulator having an automatic equalizer which regenerates an original waveform by removing intersymbol interference and phase fluctuation included in a received waveform.
Conventional demodulators for the above application include one having a decision feedback equalizer, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 36538/1982. The decision feedback equalizer has a pair of complex transversal filters respectively assigned to the feedforward side and the feedback side of a demodulator for removing intersymbol interference. A decision section is provided between the feedforward side and the feedback side for estimating a transmitted signal. A phase control section gives a phase rotation opposite in phase to the phase fluctuation base on the output of the decision section while following phase jitter and frequency offset. A tap coefficient control section updates the tap coefficients of the complex transversal filters, as needed. Data propagated through the feedforward transversal filter is added to the output of the feedback transversal filter and then fed to the decision section. The decision section estimates a transmitted signal on the basis of a set of predetermined desired signals and feeds back the estimated signal to the feedback filter. At the same time, the decision section determins a difference between the input signal and the output signal and delivers the resulting error to the phase control section. At this instant, the error includes a fluctuation in the phase of the received signal. Having a second order PLL (Phase Locked Loop) circuit including two integrating loops, the phase control section gives a phase rotation associated with a phase jitter component of the phase fluctuation of the error by the first integration loop while giving a phase rotation associated with a frequency offset component by the second integration loop. The two phase rotation is fed to a multiplier which follows the adder for adding the outputs of the transversal filters. The multiplier multiplies the phase fluctuation of the received signal and the phase rotation opposite in phase thereto and fed from the phase control section to deliver data free from the phase fluctuation to the decision section. Since the data to be fed back to the feeback transversal filter via the decision section does not include any phase component, a multiplier is connected to the output of the feedback filter for multiplying the data by a phase output corresponding to a phase fluctuation resulted from the complex conjugate of the phase rotation output by the phase control section.
In the above construction, the feedforward transversal filter removes a distortion preceding the center peak value of a distored pulse, i.e., precursor while the feedback transversal filter removes postcursor following the center peak value of the pulse. The resulting outputs of the two filters are added together and then provided with the phase rotation from the phase control section. As a result, the received data is approximated to data estimated on the basis of a set of desired signals to thereby regenerate the transmitted signal. The tap coefficients of the transversal filters are sequentially updated by a tap coefficient control section to remove intersymbol interference associated with the received signal. The tap coefficient control section is usually constructed to update the tap coefficients by use of a particular algorithm referred to as an LMS (Least Mean Square) algorithm. Specifically, with the LMS algorithm, the control unit updates the next tap coefficients such that the square of a difference between the output of the transversal filter and an estimated signal at each decision timing becomes minimum.
However, the problem with the tap coefficient updating system using the LMS algorithm is that when the transmission channel is of the kind sharply changing the characteristic thereof, the convergence rate of the system is too slow to follow the changes in the channel characteristic. It is likely, therefore, that the conventional equalizer fails to remove both of intersymbol interference and phase fluctuation.